The present invention relates to an optical memory which is capable of storing and retrieving information by light exposure. More specifically, the invention relates to an optical recording disc comprising a recording layer that has optical property which changes when the layer is irradiated with a laser.
For storing and retrieving information, an optical disc comprising a layer or film of semiconductor materials is widely used in industrial and consumer recording systems such as video recorders, digital audio recorders and document digital recorders. A signal is recorded on the optical disc by applying a laser beam so that bits of information are formed in a layer of semiconductor materials. The optical disc permits direct read after write and random access to the recorded information.
Generally, a layer of semiconductor material, such as Se, Ge, Te and InSb, is well-known for its ability to assume two stable states, amorphous and crystalline. The concept of an optical memory based on the amorphous-to-crystalline transition of the semiconductor materials mentioned above is disclosed by S. R. Ovshinsky et al, "Reversible Structural Transformations in Amorphous Semiconductors for Memory and Logic," METALLURGICAL TRANS. 2:641-45 (1971). These semiconductor materials, however, are chemically unstable and are gradually corroded in the air when they are made into a thin film. Therefore, they are not used in practice as the recording film of an optical disc memory.
Since 1971, substantial effort has been expended to produce a durable recording film of semiconductor materials. For example, in "Reversible Optical Recording in Trilayer Structures" APPL. PHYS. LETT. 38:920-921 (1981), A. E. Bell et al disclose an erasable optical recording medium based on the amorphous-to-crystalline transition of pure tellurium. The optical recording disc of Bell et al includes silicon dioxide capping layers formed on the both sides of the tellurium film to protect the pure tellurium film from corrosion and to prevent the tellurium from evaporating. In the resulting trilayer structures, the thickness of each layer must be precisely controlled, making the film forming-process unduly complicated.
Another example of an optical disc is reported by M. Takenaga in Proceedings of the 116th Study Meeting of No. 131 Film Conference of the Japan Society for the Promotion of Science, May 20, 1983, at pp. 21-26. This optical disc is based on the reversible transition of a tellurium sub-oxide thin film which is formed as a deposition layer by simultaneous evaporation of TeO.sub.2 and Te that contains Ge or Sn as an impurity. Therefore, it is difficult to control the quality of the film produced via the disclosed method, because the method includes decomposing unstable TeO.sub.2 at a high temperature. Moreover, the tellurium sub-oxide thin film had a low reflectivity (about 15%) and a low rate of reflectivity change (about 12%), resulting in a low signal-to-noise (SN) ratio.